The present invention relates to an improvement in trace element analyzers utilizing a plasma and used in material and biological sciences for quantitatively determining a trace element such as a plasma source mass spectrometer and a plasma emission spectrometer, and more particularly to an improvement in a plasma generator which utilizes microwave discharge and is used as the plasma sources of the above-mentioned trace element analyzers.
An example of a conventional microwave induced plasma source is described on pages 583 to 592 of Spectrochemica Acta, Vol. 37B, No. 7, 1982. FIGS. 2A and 2B show the structure of this example. In FIGS. 2A and 2B, reference numeral 1 designates a coaxial cable connector for applying a microwave, 2 a microwave coupler, 2' a tuner for the coupler 2, 3 a tuner for adjusting the length g of a gap between the tip of an inner tube 3' and a thin plate 4, 5 a tuner for adjusting the length of a cavity 6, 6' the wall of the cavity 6, 7 a quartz discharge tube, 8 a sample gas, and 9 an inlet for a cooling gas (for example, air).
This plasma source can be used for analyzing a gaseous sample, but does not pay sufficient attention to the analysis of a liquid sample. Thus, there arises a problem that the kind of a sample to be analyzed is limited. Further, the above example has problems that a sample introduction efficiency is low and the ionization efficiency of an introduced sample is also low.
In more detail, as is apparent from FIGS. 2A and 2B, microwave power for producing a plasma is supplied to the cavity 6 through a coaxial cable. Hence, the microwave power supplied to the cavity is 500 W at most, and it is impossible to analyze a liquid sample directly. Moreover, a large power loss is generated in the coaxial cable. Further, the coupler 2 has a complicated structure, and it is not easy to adjust the coupler 2.
Additionally, the plasma formed in the above example is based upon a surface wave. Hence, it is impossible to generate a plasma having the form of a doughnut, sufficiently. Further, the mixture of a sample and a plasma gas is supplied to the discharge tube. Accordingly, the sample introduction efficiency is low, and the ionization efficiency of the introduced sample is also low. Thus, the detection limit of a trace element (that is, sensitivity for the trace element) is low.